Agricultural sprayers use nozzles for spraying a liquid which may be a fertilizer, a pesticide, a fungicide, an insecticide onto agricultural crops. Traditional nozzles consist of an orifice with geometry controlling the flow rate, droplet size and spray pattern to the target. The flow rate through the orifice is mainly a function of the orifice area and geometry as well as the fluid pressure at the orifice (i.e., pressure just prior to the orifice). Since the orifice size is fixed, i.e., the orifice geometry doesn't change, the most common way to influence the flow rate through the nozzle is by changing pressure.
Changing the fluid pressure at the nozzle to influence flow rate changes has become common place on sprayers in order to allow for variable vehicle speed. Systems change the flow rate proportional to the vehicle speed in order to keep the application rate the same.
However, using the traditional fixed orifice nozzle has some limitations. The pressure versus flow relationship is a squared function. To double the flow requires increasing the pressure by a factor of four times. Unfortunately, changing pressure also changes atomization dynamics resulting in an impact on spray quality. Spray quality characteristics, namely, droplet size and the spray angle, both become smaller as pressure increases. These changes can negatively impact spray deposit and spray drift. So, the need for a variable rate nozzle with uniform pressure has emerged.
In recent years, pulse width modulation (PMW) has been used to control nozzle flow rate while maintaining constant pressure. The idea uses a solenoid coil and armature valve configuration to open and close flow to the nozzle. The valve is cycled between open (full pressure) and closed (no pressure) positions at a fast pulse rate. Changing the effective flow rate is done by varying the portion of open time to the portion of closed time. So, if a nozzle that sprays one gallon per minute is open only 50% of the time, the result will be an effective flow rate of only a half gallon per minute. In this arrangement, the duty cycle of open time is a linear relationship with effective flow rate. As the vehicle speed diminishes, the duty cycle decreases to match the flow rate required, while maintaining a constant pressure. U.S. Pat. No. 5,134,961 discloses a nozzle arrangement of this type which modulates nozzle flow by intermittent operation of an electrical solenoid valve attached to and located directly upstream of a spray nozzle. By cycling the valve open and closed, the flow rate through the nozzle is controlled in an analogous on/off manner.
However, the PWM technology has some inherent problems. Poor coefficient of variation in the travel direction (up to 65% in one study) is caused as a result of turning the flow to the nozzle “off” and “on” during travel. It is known in the art to pulse the nozzles at 10 Hz and alternate phase 180° on every other nozzle to blend the spray. A faster pulse is recommended to help with the coefficient of variation. But, the open/close response time of the solenoid armature poses a limitation to pulsing faster. Thus, it is desired to find a way to pulse faster so as to improve the coefficient of variation.
One further drawback of using a pulsing solenoid-operated valve to control flow to the nozzle is that the electrical current draw is rather high and requires a sprayer vehicle capable of handling high electrical current loads.